USGS Bulletin 2130 -- CHAPTER 14

SLIDEQUAKES AND FAULT CREEP AT THE SLUMGULLION LANDSLIDE: AN ANALOG TO CRUSTAL TECTONICS

Fleming and Johnson (1989) noted that structures observed on certain types of landslides are strikingly similar to those associated with crustal-scale tectonics (fig. 1). This surficial similarity suggests the hypothesis that some landslides may provide useful analogs for the study of processes involved in crustal-scale tectonics, thus bridging the gap between laboratory experiments and regional field studies. We present observations of geomorphic and geophysical expressions of landslide faulting that support this hypothesis. The benefits of landslides as models are evident in the fact that much of the data presented was collected within a week on the Slumgullion landslide using conventional geophysical instrumentation from an area spanning less than 0.5 km². These results, in addition to further demonstrating the analogous behavior of landslide and crustal faulting, suggest several new opportunities for monitoring and evaluating landslide deformation and the hazards that landslides pose.

This experiment was designed to observe indicators of slide rheology and modes of deformation. In particular, we hoped to determine (1) if the displacement of landslide material, which had previously been observed to occur primarily along faults in and bounding the landslide (Crandell and Varnes, 1961), occurs along discrete or distributed shear zones; (2) whether landslide fault-slip occurs seismically or as aseismic fault creep; if slidequakes exist (indicating brittle failure), then (3) how does the rate and distribution of seismicity and creep correlate with the rate of fault slip?

To address these questions, during the period June 22-28, 1993, we deployed and operated a buried, digital, high-precision creepmeter and a portable seismic network in the vicinity of one of the major strike-slip faults bounding the Slumgullion landslide. Relative velocity vectors were also measured using Global Positioning System (GPS) methods. Measurements made over several months included creep observations from a modified tide-gauge used as a wire extensometer and from the displacement field in a grid of stakes placed across the fault. Both sets of measurements were made in the same region covered by the seismic network (fig. 2). Herein, the seismic, creep, and stake-grid measurements and their implications are summarized. GPS measurements and implications are summarized in chapter 15.

The displacement of the stake grid shows that deformation in the immediate vicinity of the slide-bounding strike-slip fault occurs as block motion (fig. 3A). Essentially rigid-block motion is also observed across segments of crustal strike-slip faults (fig. 3B), the San Andreas and Calaveras faults, which accommodate plate motions principally by continuous creep (fig. 4 in Savage and Burford, 1971; Lisowski and others, 1991).

Results of analyses of portable seismic network data suggest that slidequakes exist at the Slumgullion landslide and are detectable with conventional instruments. Slidequakes are evidenced by short duration, spatially and temporally clustered signals (fig. 4A), with sources that most probably occurred on the slide-bounding strike-slip faults (fig. 5). The seismic network that recorded these slidequakes included four analog seismographs and a phased digital micro-array. The analog seismographs operated with single-component sensors synchronized daily to radio-broadcasted time. Drift rates of the analog seismographs' internal clocks were insignificant relative to the precision with which seismic phases could be timed. Inside the analog network and crossing the slide-bounding fault, a phased digital micro-array contained three single-component sensors spaced at 50 m from a central three-component set of sensors (fig. 2). The digital signals were recorded on a single seismograph synchronized continuously to radio-broadcasted time. The relative arrival times at each station for 13 of the best recorded slidequakes show that they originated in essentially the same location. We were therefore able to treat them as one event, with little loss of information, and used the average relative arrival times to determine the approximate location of the events. The non-impulsive phase onsets and slightly dispersed wavetrains of these signals and ray-tracing analyses indicated that the observed signals were surface waves. Hypocenter estimation requires knowledge of the seismic velocity structure. Arrival-times measured for P-wave and S-waves on seismograms of four explosions guided trial-and-error ray-tracing to estimate the velocity structure. The resultant P-wave and S-wave velocities indicate that the Poisson's ratio within the slide is ~0.49. Such a high value is consistent with models of slide deformation in which the slide material behaves plastically on some time scales (Savage and Smith, 1986).

Long duration, sinusoidal-like signals were also observed (fig. 4B). These signals may originate from slide-generated sources, but their generation by non-natural sources, such as traffic, cannot be ruled out. The very emergent onset of these signals makes timing of their onsets and estimating their hypocenters extremely difficult. Although they were observed most commonly at stations closest to traffic, in several instances the most distant stations recorded the largest amplitudes. The frequency of occurrence of these signals and time-of-day did not clearly correlate, and thus their association with traffic was ambiguous (traffic was relatively minimal during the night). Several sources may give rise to these monochromatic signals, some natural and from the slide and others man-made. Possible sources include slow rupture of faults or materials entrained within the faults (e.g., trees, boulders), or slow basal slip along the slide-bedrock interface.

Figure 5 shows a map-view distribution of seismic-source-location probability confidence levels at the ground surface. The most probable location of slidequakes is seen to occur in the vicinity of the southern slide-bounding strike-slip fault.

Creep observations presented by Savage and Fleming (this volume) for the Slumgullion landslide's south-bounding strike-slip fault show seasonal variations around a steady rate of ~1.5 cm/day (fig. 6A). We note that the creeping section of the San Andreas fault also exhibits seasonal variations (fig. 6B). The change in slope in both cases corresponds to increased climatic moisture: fall rains along the San Andreas fault and the spring snow melt on the Slumgullion landslide. Moreover, "creep events" were recorded that were triggered by explosions set off several tens of meters from the fault (fig. 6D). These explosions were used to calibrate the velocity structure for the seismic data analysis. It was not possible to assess whether these triggered creep events were seismogenic because their signals would be obscured by those generated by the explosions. Creep events triggered by earthquakes (fig. 6C) have been documented along the San Andreas fault but are not well understood (Bodin and others, 1994). The observation that creep events can actually be created along the landslide fault suggests future controlled experiments of triggered creep. Both the observation of slidequake signals and of triggered creep events implies that, at least on short time-scales, the landslide material behaves elastically, storing and releasing elastic strain energy.

Our results illustrate that probable slidequakes can be recorded using conventional instrumentation and simple installations. Slidequakes may occur more frequently than our observations indicate because the small amplitudes of the slidequake signals were barely discernible above the background noise and only during periods of no wind (the roots of swaying trees and shrubs generate seismic signals). Their abundance could be more accurately characterized by use of sensors that have a broader frequency range (slidequakes may emit seismic waves with frequencies outside the range of the recording instruments) and that were buried deeper to reduce wind-generated noise.

Although the measurements only spanned a short time on a single landslide, analytical results suggest that landslide deformation occurring along the slide-bounding fault is analogous to creeping crustal faults. In particular, an analogy can be drawn to the San Andreas fault zone because it is the only fault in which both creep and seismicity are well documented. A segment of the San Andreas fault zone between San Juan Bautista and Cholame creeps at the fault's long-term slip rate (Savage and Burford, 1971), indicating that accumulating strain is continuously relieved almost completely by steady-state slip. In addition to a nearly constant slip rate, this creeping section of the San Andreas also exhibits a high rate of small-magnitude seismicity. The relationship between the production of small earthquakes and creep is not well understood. Observations of creep and seismicity probably associated with slip along the landslide's strike-slip fault suggest that similar processes may be operative on both the Slumgullion and San Andreas faults. The accessibility in three-dimensions and the potential to do controlled experiments (e.g., to create creep or seismic events) along the landslide fault make it an ideal natural laboratory for study of crustal faults. Moderate earthquakes have ruptured through creeping sections of the San Andreas fault, and the possibility that larger slidequakes may also occur and (or) that other sections of the landslide fault exhibit differing behaviors cannot be dismissed.

We have demonstrated that quantitative measures of landslide deformation, such as seismic, geodetic, and creep observations, can be made in a short time with readily-available instrumentation. Comparison of these observations to those recorded near crustal faults that require more complex instrumentation deployed for much longer time periods, suggests that much could be learned from future studies of landslide faulting. Our results indicate that the displacement of landslide material appears to occur along discrete faults exhibiting a combination of brittle failure (evidenced by slidequakes and creep events) and stable sliding. The manner in which the rate and distribution of seismicity and creep correlate with fault-slip rate remains to be determined in future experiments.

The authors thank M. Meremonte, E. Cranswick, T. Bice, D. Overturf, P. Powers, T. Pratt, R. Williams, and J. Savage for their assistance in the field experiment and B. Kindel for his help in analyzing the seismic data.

Accessibility FOIA Privacy Policies and Notices

U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubsdata.usgs.gov/pubs/bul/b2130/Chapter14.html
Page Contact Information: GS Pubs Web Contact
Page Last Modified: Friday, 23-Mar-2012 14:21:29 EDT